Multilaser
Multilasers are the standard armament of the Imperial Guard Chimera transport. They're basically a Hellgun modded with multiple barrels and bulked up to the point where it's firing a fairly powerful beam with a high firing rate. It's a pretty good weapon against just about anything that isn't heavy armor (especially infantry with shitty armor, light vehicles, and skimmers), though like most Imperial Guard anti-infantry weapons, the Multilaser's armor-piercing capability is pretty weak. It's still a good enough weapon to threaten squads with, which is a big reason that it's the weapon of choice for Imperial Guard APCs, Walkers, and some Valkyrie assault craft. But mostly Chimeras, due to being an excellent suppression weapon to cover dismounted troop squads with (not that the game has mechanics for suppression except from sniper fire). In the game Fire Warrior, they are also occasionally used as weapon emplacements. To help you visualize what a multilaser is like, we point you to the gatling laser of Fallout fame, then imagine it bigger, higher power, and lower rate of fire more similar to a machine gun. Since it has such high power, but low armour piercing, it probably fires multiple beams at the same time or wide dispersed beams, or either depending on the specific model. If there was a rule 63 (solid-projectile) counterpart, it would be something like a machinegun-like Assault Cannon knock-off or a Heavier Stubber with explosive rounds.
Multilasers and C.S.Goto
The Multilaser is, by and large, unique to the Imperial Guard; sadly this has not stopped a certain Black Irish leper from completely disregarding everything remotely resembling canon and cramming them onto absolutely everything due to his raging hardon for the damned things. As of so far, Goto has managed to rape canon in more than a dozen ways using this weapon alone, including giving them to Space Marines and mounting them on Land Raiders (and not out of desperation either, this was the chapter's normal crusade-ready loadout). His love for them reached memetic levels, to the point some people joked that a Carnifex picked one up in his books, and after a while people actually thought the joke was true (after all, nobody wanted to give the leper money just to check for themselves).
And now Forge World has jumped on the bandwagon by letting Marines in 30k take Rotor Cannons, which are Multilasers but somehow worse. Q: Aren't Rotor Cannons projectile-based weapons that's the precusor to the Assault Cannon (and basically a weaker version of the heavy stubber) instead of energy-based weapons like the Multilasers?